Image sensors can be realized with semiconductors based on their capability to convert locally impinging light energy into a proportional amount of electronic charge. This electronic charge “Q” is often referred to as photo-charge, and can be integrated within the pixel on a storage device such as a reverse biased diode or as a pre-charged metal-oxide-semiconductor capacitance. The finite charge storage capacitance within each pixel limits the amount of integrated photo-charge. Dynamic range is measured as the ratio of the maximum photo-charge that can be meaningfully integrated in a pixel of the imager to the pixel noise level.
Intrascene dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. Examples of high dynamic scenes range scenes include an indoor room with a window view of the outdoors, an outdoor scene with mixed shadows and bright sunshine, and evening or night scenes combining artificial lighting and shadows. In a typical charge coupled device (CCD) or CMOS active pixel sensor (APS), the available dynamic range is in a range of 1,000:1 to about 4,000:1. Unfortunately, many outdoor and indoor scenes with highly varying illumination have a dynamic range significantly greater than 4,000:1. Image sensors with intrascene dynamic range significantly greater than 4,000:1 are required to meet many imaging requirements.
The dynamic range of an image sensor can be increased by using multiple exposure times and/or integration times. For example, U.S. Pat. No. 4,647,975 describes a method based on the acquisition of two or more images, each having an exposure time. Once numerous images have been taken at different exposure times, the images have to be fused or merged to form one single piece of pixel information having a wide dynamic range. U.S. Pat. Nos. 4,647,975, 5,168,532, and 5,671,013 disclose the use of a selection rule to combine information from the most suitable of the multiple images. The merged pixel information or value is then multiplied by a suitable factor that corrects for the respective exposure times. This method however exhibits undesirable temporal aliasing if the scene or camera is moving because the two or more images having different exposure times are captured using the same image sensor and thus are not captured concurrently.
Despite improvements in solid-state image sensor and digital camera technology, the light signal or brightness range of scenes often exceeds the dynamic range of the sensor. For this reason, numerous methods have been described in the art of image sensors to extend the dynamic or signal range. A summary of some methods is presented in: “Wide dynamic range sensors”, Optical Engineering, Vol. 38, No. 10, pp. 1650-1660, October 1999. Methods to provide wide dynamic range imaging capability with a single image sensor include: (a) logarithmic or compressed response photo-detection; (b) multiple integration and charge storage capability within each pixel; (c) frequency based sensors, where the sensor output is converted to pulse frequency; (d) local integration time control, where different areas within the sensor can have different exposure times; (e) signal charge versus integration time rate (signal slope) measurement; (f) analog to digital conversion per pixel; and (g) autonomous pixel control. These methods require complex pixel circuitry and are difficult to implement in small pixel areas without taking up area required for the photo-detection mechanism (such as a photodiode).
Typical scenes imaged by digital cameras have light levels that span a range including low light (1-100 lux), moderate light (100-1000 lux), and bright light (1000-1,000,000 lux) under outdoor conditions. To accommodate lighting changes from scene to scene (the interscene dynamic range) an electronic shutter is used to change the integration time of all pixels in an array from frame to frame. To cover a single scene that might involve indoor lighting (100 lux) and outdoor lighting (100,000 lux), the required intrascene dynamic range is approximately 10,000:1, corresponding to 80 dB (14-bits). This exceeds the dynamic range of a single image sensor using a single integration time (typically 3,100:1 corresponding to 70 dB (12-bits)). Therefore, there is a need for a digital camera in which the effective single-frame dynamic exposure range is expanded.